Terahertz (THz) systems have become important for a wide variety of applications in analysis, diagnostics, and potentially communications. A large number of significant technical innovations over the last decade has made the terahertz frequency range (0.1 to 10 THz) increasingly accessible for applications in science and industry. However, difficulty in generating, manipulating, and detecting terahertz radiation continues to plague most applications. Source powers (from sources that are readily attainable) tend to be low. Focusing optics is costly, difficult to align, and has high loss. Detectors are expensive, inefficient (particularly at high frequencies), and usually require alignment with an optical pump source. In combination, these factors restrict terahertz applications to exploratory and scientific investigation, rather than to what could become large markets and relatively high-volume applications.
One of the most common applications for THz technology is spectroscopy. In spectroscopy, the frequency dependence of transmission of electromagnetic radiation through a sample reveals a unique fingerprint of substances being studied. In the THz frequency range, this fingerprint results from unique rotational and vibrational energy states associated with complex molecules, providing information related to composition and conformal state. While other approaches (e.g. Raman spectroscopy) offer views into molecular form and function, THz spectroscopy provides a unique and complementary view. As a result, an easy to use and cost-effective THz spectrometer will stimulate environmental, medical, material science, and other applications.
Both time-domain spectrometers (TDS) and frequency-domain spectrometers (FDS) are well established. At the heart of each, as shown by the dotted lines in FIG. 1, is a THz generation-analysis-detection (TGAD) module 100. In a typical TDS, a short laser pulse (50 fsec) is split and focused on separate transmit and receive photomixers (PM) 110. The terms photomixer and photoconductive switch are generally used in reference to FDS and TDS operation, respectively. As used herein, the term photomixer is applicable to both modes of operation. The transmit PM is biased such that the change in conductivity induced by absorption of the short laser pulse creates a current pulse that drives a small dipole antenna, radiating THz frequencies that are captured by a silicon lens 120 to produce the diverging THz beam shown 130. A TEFLON polytetrafluoroethylene lens 140 then creates a collimated THz beam 150 that is used to probe the sample. After passing through a sample, the THz beam 150 is focused to the receive PM 110 where it is sampled by the short laser pulse to produce a small current as measured typically with a lock-in-amplifier. The sampling time is adjusted using a variable optical delay, in this case introduced by varying a retro-reflector. FDS operate in the same manner except that instead of using a short laser pulse, two continuous-wave lasers separated in optical frequency by the THz frequency are combined to produce the THz pump intensity variations.
Many applications for THz analysis have been defined [1], including measurement of gas-phase samples, liquids and solids. Examples include:
Breath analysis: Non-invasive diagnostics of disease: Detecting volatile compounds in breath has gained considerable attention for medical diagnosis [2] due to its non-invasive nature and the potential for breath-by-breath analysis. Numerous compounds in exhaled breath are valuable indicators of an individual's health status [3]. The Federal Drug Administration (FDA) has approved some compounds for breath testing, including ethanol (C2H5OH) for law enforcement, hydrogen for carbohydrate metabolism, nitric oxide (NO) for asthma, carbon monoxide (CO) for neonate jaundice, 13CO2 for H. pylori infection (related to stomach cancer and normally asymptomatic), and branched hydrocarbons for heart transplant rejection. Additionally, increased breath ammonia (NH3) is found to relate to kidney and liver dysfunction, breath acetone is higher in diabetes, and the level of aldehydes such as methanol (CH2O) can be used to screen lung and breast cancers. Currently applied analytical instruments include mass and mid-IR spectrometers. Although these are large and expensive, both are in widespread use, supporting diverse instrumentation and diagnostic industries. THz spectrometers can record fast processes, opening up a unique potential for real-time analysis of exhaled air.
Security: Several applications exist in the area of explosive detection, where flames, plumes, and explosive vapour are of great interest. Collective motions in molecules found in common explosives correspond to features in THz spectra that can be used for unique identification. There have been extensive studies on the THz spectroscopy of explosives like DNT, RDX, HMX, TNT, and PETN [4]. Also, understanding combustion requires knowing the species present and the spatial distribution in the flame, as measured using THz in [5].
Environmental: Rotational transitions of light polar molecules and low-frequency vibrational modes of large molecular systems can be probed by THz spectroscopy, opening applications in sensing atmospheric pollutants and detecting airborne chemicals. Atmospheric pollutants like hydrogen sulphide (H2S), OCS, formaldehyde (H2CO), and ammonia (NH3) possess intense THz transitions [6]. Volatile organic compounds (VOCs) are of high interest in manufacturing and oil and gas industries, and are potentially detectable in real time using the proposed instrument.
Scientific: Large numbers of applications have been considered within the laboratory, including studying the absorption and dispersion of compounds [7], dynamics of laser induced plasmas [8], real-time trace gas detection [9], and the analysis of chemical compositions [10].
THz generation-analysis-detection (TGAD) modules used in existing THz spectrometers (FIG. 1) use discrete optical components to form free-space THz beams. These suffer from mechanical, performance, and cost challenges. Mechanically, they are bulky, require difficult alignment and are subject to mechanical instability. Several factors limit performance. Significant restrictions are placed on the diameter and length of the interaction region and on the achievable electric field strength within this interaction region. Given the size and the need for adjustment, it is difficult to isolate measurements from external factors, such as water vapour that must be removed by purging with dry gas. Most importantly, numerous elements in cascade introduce substantial loss, including frequency dependence of the antennas, reducing dynamic range and bandwidth. Several factors combine leading to high cost, led by the required optical (both THz and pump laser) components and alignment. Hence there is a compelling need to improve upon the state of the art.
Confining THz radiation within waveguide structures offers tremendous advantages in size, performance, and versatility, driving research on many types of THz waveguides, such as coplanar strip lines [11], metal pipes [12], dielectric fibers [13], etc. The single-wire waveguide [14] shows low loss and low dispersion, but it is difficult to couple the output from a typical PM to the radially-polarized mode supported by this waveguide.
Two-wire waveguides [15,16] combine both low loss and efficient coupling properties. The mode supported by this type of waveguide is very similar to the field emitted from a simple dipole. For the TEM mode there is no cutoff frequency and no dispersion. Confining electromagnetic energy in a small area between the two wires is another important advantage, making it more practical and more tolerant to bend loss [17].
For low-loss transmission over the terahertz band, the two-wire waveguide described above has a mode area of typically 20 mm2. For THz operation, active components like photomixers have very small active areas (e.g. 20 μm2)—much smaller than the mode area of the low-loss passive structures described above. Also, unlike the passive components described above, active components are fabricated in-plane, typically on III-V semiconductors. Therefore, some technique for matching these very small active devices to these much larger waveguide structures is required.
Disclosed herein are methods and apparatus that permit efficient transitions from small and active components like a photomixer to a larger waveguide structure. It is anticipated that such transitions will be essential in making THz measurement affordable and easy to use, improving performance, simplifying alignment and adding mechanical stability. In addition, the novel methods disclosed for coupling between THz sources and THz waveguides will have broad application in other areas, such as communications.
In some examples, systems for transmission of terahertz signals comprise at least one terahertz device configured produce a terahertz electrical signal, and a terahertz waveguide operable at terahertz frequencies configured to transport the terahertz electrical signal. A mode-matching taper is situated so as to couple the terahertz device to the terahertz waveguide and direct the terahertz electrical signal from the terahertz device to the terahertz waveguide. In some embodiments, the mode-matching taper comprises a substrate and a first tapered waveguide section situated on the substrate so as to substantially match a component of a propagating electrical mode associated with the terahertz electrical signal produced by the terahertz device. In further examples, the mode-matching taper further comprises a second tapered waveguide section situated on the substrate so as to substantially match a component of a mode supported by the terahertz device to a mode associated with the terahertz waveguide. In some examples, the substrate comprises a planar surface and the first and second waveguide sections are situated on the planar surface. In representative embodiments, the first waveguide section is defined by at least one conductor situated on the planar surface of the substrate and includes an in-plane taper at the planar surface so that the first waveguide section is associated with a waveguide mode that substantially matches an in-plane component of the electrical mode associated with the terahertz electrical signal, and the second waveguide section includes a taper normal to the plane of the surface so as to substantially match the component of the electrical signal produced by the terahertz device normal to the planar substrate and a mode associated with the terahertz waveguide. In other representative examples, a cap layer is situated on the planar surface, and the substrate and the cap layer are at least one of silicon, GaAs, or InGaAs. In other embodiments, the substrate includes a thinned portion situated along at least a portion of the mode-matching taper or substrate is a one-dimensional photonic crystal. In one embodiment, the terahertz waveguide is a two wire waveguide. In other examples, the mode-matching taper includes at least one tapered section corresponding to a tapered slotline, a tapered coplanar stripline, a tapered microstrip, a tapered stripline, or a tapered coplanar waveguide. In some embodiments, the mode-matching taper includes a first section defined by a tapered slotline having a taper in a first direction and a second section defined by at least two tapered wires, wherein each of the two wires is tapered in a second direction that is perpendicular to the first direction.
According to other examples, mode-matching tapers for coupling between a terahertz device and a terahertz waveguide include a device port configured to couple to a terahertz device and a waveguide port configured to couple to the terahertz waveguide. A tapered waveguide is situated so as to connect the device port and the waveguide port, wherein a dimension of the tapered waveguide transitions from a dimension associated with the device port to a dimension associated with the waveguide port. In other examples, the tapered waveguide includes a plurality of steps or continuously so as to transition from the dimension associated with the device port to the dimension associated with the waveguide port. In some examples, the waveguide port is configured to couple to a two wire transmission line. According to other examples, the tapered waveguide is defined on a substrate and a terahertz device is formed at least partially in the substrate. In additional examples, a cap layer is situated on the substrate and configured to suppress terahertz wave coupling into the substrate based on a thinned region of the substrate situated at the tapered waveguide, or with a substrate that is a one-dimensional photonic crystal.
Representative methods include transmitting a terahertz electrical signal with a terahertz waveguide and coupling the terahertz electrical signal to terahertz device with a mode-matching taper situated between the terahertz waveguide and the terahertz device. In some examples, the terahertz device is a terahertz generator or a terahertz detector. In other examples, the terahertz electrical signal is directed from the terahertz waveguide to a specimen, and the terahertz electrical signal is detected after interaction with the specimen. In further embodiments, the terahertz electrical signal is generated with a pulsed optical beam or based on a combination of two or more optical beams having a terahertz frequency difference.
These and other features and aspects of the disclosed technology are described in further detail below with reference to the accompanying drawings.